Why not? They could have used the fan favourite Nightcrawler who almost everyone has been asking, same as Kelsey's Beast, or even Gambit. Since the movie is set many years after X3, they wouldnt need to explain why some characters are there and why others arent. But they have confirmed curiously 5 of the characters from the comics. It isnt just a coincidence.

Matthew, Singer and Kimberg could have left Kitty out, since Ellen only appear on one movie, as some fans said many times these past months, but they decided to included her. It should be for a reason, the biggest one because she is a pivotal role on the storyarc, and many fans would have been pissed if she was out of this big sequel. We could say the same as Colossus, since he was just a large cameo on the trilogy.

-Nightcrawler didn't appear in X3, if he appeared in X3 just like Colossus and Shadowcat. Singer would have probably already announced his comeback.
-Gambit wasn't part of the original trilogy and First Class.
-Beast joined the United Nations and it looked he left the X-Mansion and moved to DC.

Its obvious they aren't picking characters just because they are "fan-favorites". Watching the ending of X3, you can tell which mutants are going to comeback if they ever make a sequel and those are Wolverine, Storm, Rogue, Iceman, Shadowcat and Colossus. And right now, all of them were already announced to comeback.

And this excuse of "oh its been years since First Class and X3, they wouldn't need to explain why they disappeared or why they cameback" is not good story-telling.

If you think they are really picking characters based on the comic-book version of DOFP, why haven't they announced an actor who could potentially play Rachel Summers and Franklin Richards. This movie is NOT gonna be a 100%% adaptation of the comic-book.